Writer: How can I convert all fields in a doc to fixed content?

I see an “Fixed content” checkbox under “Insert fields” but not a way to apply this to all existing fields.

Version: 25.2.5.2 (X86_64) / LibreOffice Community
Build ID: 03d19516eb2e1dd5d4ccd751a0d6f35f35e08022
CPU threads: 4; OS: Windows 7 Service Pack 1 X86_64 (6.1 build 7601); UI render: Skia/Raster; VCL: win
Locale: en-GB (en_GB); UI: en-US
Calc: threaded

Thank you.